The invention relates to a process for the conversion of ground hydrous lignite into a pumpable dehydrated suspension of fine-ground lignite and oil. Suspensions of this kind are used for subsequent hydrogenation at temperatures of 300.degree. to 500.degree. C. and pressures of 100 to 700 bar.
Hydrogenation of lignite is a known process which was developed and perfected for industrial application by Bergius and Pier.
The lignite is prepared for hydrogenation by grinding and drying to a water content which should be less than 5% and by mixing it with oil, thus obtaining a suspension. A higher water content would have an unfavorable effect on hydrogenation and on the pumpability of the lignite/oil suspension.
The relatively high pit humidity of the lignite, i.e. 50 to 60% water, referred to coal, requires a considerable expenditure for dehydration. In the literature hitherto published, two methods of dehydration have been described. One method consists of drying the lignite with low-pressure steam or hot flue gases and another method comprises the mixing of the lignite with oil with subsequent removal of the water by distillation as referred to in the Canadian Pat. No. 978,877. Dehydration by evaporation is very expensive as regards energy and equipment and furthermore influences the thermal efficiency of the hydrogenation process. Removal of the water by distillation, which requires very large heat exchange surfaces, is hardly feasible from the economical and industrial standpoint.
Another method, a further development, as described in DP-OS Nos. 26 53 033 and 28 31 024, consists of mixing the pit-wet lignite with oil and dehydrating the mixture at high temperatures and high pressures. The disadvantage of these methods resides in the poor flowability of the hydrous lignite/oil sludge. The sludge transport through pipes and heat exchangers presents considerable difficulties. Another disadvantage is the relatively high expenditure for the equipment which is necessary for heating the suspension to temperatures above 250.degree. C. and for its subsequent cooling. A further disadvantage is the three-phase separation of oil/water/oil-soaked lignite which cannot be achieved by simple sedimentation of the phases.